There are myriad instances in which it is necessary to precisely align two objects. For example, an electronic component to be placed on a carrier such as a printed circuit board must be aligned to within very tight tolerances with the conductors on the circuit board, before it is placed. In the same industry, it is also necessary to align a stencil with a printed circuit board when applying solder paste to the board. These are but two examples of this alignment need.
The object alignment in such instances can be accomplished using direct vision, either unassisted or with assisting optics ranging from a simple magnifying glass to sophisticated microscopes. Direct camera vision may be used to assist the operation. More sophisticated methods employ superimposition or juxtaposition of images of the objects on a video monitor. Typically, the two images are created using a single video camera or CCD, (or perhaps two image sensors), and reflecting optics such as prisms to direct light rays from both objects to the imaging device. Examples of such imaging systems may be found in U.S. Pat. No. 5,311,304, to Monno; U.S. Pat. No. 3,988,535, to Hickman et al.; U.S. Pat. No. 4,642,813, to Wilder; U.S. Pat. No. 4,643,579, to Toriumi; and U.S. Pat. No. 4,899,921, to Bendat et al., and the Summit Series rework systems made by Sierra Research and Technology, Inc., Westford, Mass. 01886.
Direct vision alignment leads to parallax errors, and often has insufficient resolution for fine alignment needs. In addition, direct operator vision alignment is subject to operator error. Imaging systems employing reflecting optics between the image capture device and the objects being imaged are also problematic. Reflecting prisms and mirrors can cause image distortion, leading to alignment errors. Reflecting optics are also subject to thermal instability, which can also lead to alignment errors. Also, since a single image capture device is being used to capture images of two objects, each image is by definition not as bright as the entire image, making such systems more difficult to use, and more likely to result in alignment errors.
There are also imaging systems having a single camera which is moved to sequentially image the two objects. Examples include U.S. Pat. No. 4,924,304, and Re. Pat. No. 34,615, both to Freeman. Such systems require precision mechanical systems to accomplish the necessary movement of the camera or the imaging optics. Such mechanical systems are expensive, require careful calibration, and are prone to error, particularly as they wear.
Similar systems exist which use two separate imaging devices, usually oriented in parallel, or normal to one another. Examples include U.S. Pat. No. 5,194,948, to L'Esperance; U.S. Pat. No. 5,305,099, to Morcos; U.S. Pat. No. 5,113,565, to Cipolla et al.; and U.S. Pat. No. 4,855,830, to Davis et al. The use of separate sensors, however, requires careful calibration before use, and is prone to misalignment errors because the alignment is dependent on the accuracy of two separate camera positioning and moving devices; if either one wears or introduces error, the entire system will be in error.